Microgravity Science and Technology最新文献

The dynamics of a droplet on a liquid substrate in the case of an inhomogeneous heating from below has been investigated. The problem is studied numerically in the framework of the slender droplet approximation and the precursor model. The change of the stationary droplet’s shape and the rupture of the substrate layer induced by a floating droplet are investigated. The influence of the gravity force on the shape of the droplet is studied.

Fluid manipulation and control is crucial for space exploration. Motivated by the “Thermocapillary-based control of a free surface in microgravity" (ThermoSlosh) experiment (Salgado Sánchez et al. in Acta Astronautica 205:57–67, 2023), we conduct here a detailed numerical analysis of interfacial dynamics in a two-dimensional cylindrical cell, half-filled with different silicone oils or a fluorinert, and subjected to thermal forcing and vibrations. The effect on the free surface dynamics of the applied temperature difference, vibrational amplitude, fluid viscosity, and contact angle is analyzed; both static and dynamic contact angle models are considered. Results strongly suggest that thermocapillary flows can be used to control the interface orientation within the cell, while supplemental vibrations can be added to increase the system responsiveness. This control can be further improved by using classical proportional-integral-derivative feedback to adjust the cell boundary temperatures in real-time. The proportional and derivative gains of the controller can be selected to optimize the stabilization time and/or energy cost, while the integral contribution is effective in reducing the steady-state error. Overall, the present analysis highlights the potential of using the thermocapillary effect for fluid management in reduced gravity, and evaluates different types of experimental tests that can be executed in the frame of the ThermoSlosh microgravity project.

Friction is a primary failure mode in micro-nano electromechanical systems due to the high surface-to-volume ratio. Microgravity further complicates this issue in journal-bearing-like conformal contacts by promoting irregular disturbances. This paper aims to gain insights into the anti-friction design of journal-bearing-like devices through molecular dynamics simulation. A molecular dynamics model was proposed and the calculation method of the friction force was derived. In the absence of disturbance, the proposed model was compared with a non-conformal model which unfolded the bearing as a plane, and the influence of initial radial clearance and axis inclination on the friction force was investigated. The results showed that the proposed model could present more accurate friction forces than the non-conformal model. The friction force was inversely proportional to the initial clearance, and the axis inclination could reduce the friction force. Regarding disturbances as the superposition of two vibrations perpendicular to each other, in which case the trajectory of the journal was a Lissajous curve, the effects of frequency, stiffness coefficient, amplitude ratio, and frequency ratio were investigated. The results showed that the average friction force increased with the rising frequency in the range of 0.8 ~ 4.8 GHz, then decreased with the further increase of frequency. The average friction force was lowered when the stiffness coefficient increased from 100N/m to 1000N/m. For two representative frequencies, the average friction force exhibited different trends with the amplitude ratio. Except for the case of 1.25, increasing the frequency ratio could reduce the friction force. It seemed that applying a well-designed Lissajous route was a promising way to reduce friction.

Nowadays a propellant residual gauging method based on the thermal response of the tanks’ wall is developed. And the liquid distribution and meniscus height have great effects on the thermal response. Profiles of liquid free surfaces in revolved containers under microgravity are studied through theoretical analysis and numerical simulation in this paper. The analytical formula for the static profile of the liquid surface in the spherical tank is established. It shows that the profile is a section of a circle cut off by the tank wall. For given the geometry of the tank, liquid volume and contact angle, the profile of the free surfaces under microgravity can be obtained by using the Shooting method based on the theoretical model. Numerical simulation is carried out with the Volume of Fluid method, and it is verified that the static profiles at different contact angles and liquid filling rates fit the theoretical descriptions. It is concluded that the meniscus height increases slowly as the filling rate increases, and the smaller the contact angle, the more obvious this trend. Then the theory is extended to the tanks of arbitrary shapes, and the critical position of the profile is derived. Below the critical position the propellant may accumulate in some corners or pits, which makes it unable to be fully utilized. The critical position is related to the shape of the tank and the contact angle. This research is of great value for the prediction of the static profiles of liquid surfaces in tanks and the propellant residual gauging.

The problem of stability of one-dimensional filtration flow in a rectangular domain of porous medium is solved. The flow occurs when a portion of impurity is transported through the region against gravity. It is shown that the instability has an absolute character. A Rayleigh-Taylor instability is observed at the backward front of the concentration pulse. In this case, the observation time is always less than the passage time of the pulse through the domain. A theoretical model is proposed to describe this phenomenon taking into account immobilization and clogging. The influence of the problem parameters on the characteristic time of instability onset is investigated. Comparison of computational results with experimental data has shown the appropriateness of the chosen model. The ways of increasing this time are analyzed. It is shown that only one way to increase the instability time is to significantly reduce the buoyancy force impact. The latter force can be diminish by alteration of the gravity force.

Blowout is among catastrophic accidents in oil and gas drilling, and it is caused by abnormal pressure resulted from gas kick from reservoir which cannot be prevented due to limits of drilling technology. Accurate prediction of wellbore pressure is an effective method to prevent blowout. Based on electrical capacitance volume tomography (ECVT), the experiments of gas and white oil two-phase flow with viscosity of 16 mPa·s, 23 mPa·s, 26 mPa·s and 39 mPa·s in vertical annulus are carried, and the pressure drop in vertical annulus is tested. Considering the influence of viscosity, modification of the friction loss coefficient and prediction of the pressure gradient in bubble flow, slug flow and churn flow are studied. The prediction accuracy of the modified model is compared with the pressure gradient model established in the Caetano’s experiment (air-kerosene, ID 42.2 mm and OD 76.2 mm). The results show that under the Caetano’s experimental conditions and the experimental conditions of this experiment, the maximum error and the prediction mean absolute error of the pressure gradient model with the corrected friction loss coefficient are lower than those of Caetano’s model.

The dissipation behavior of granular balls in a quasi-2D closed container subjected to vertical vibration is studied by means of discrete element method in this paper. Four universal granular phases playing high damping effect are finalized by simulating the gravity environments of Earth, Mars and Moon, respectively. Based on the commonality of dense granular clusters in the four high damping granular phases, the ideal dissipation behavior of granular balls in the quasi-2D closed container is indicated. Moreover, the optimal damping mechanism of granular balls in the quasi-2D vibrated closed container is further revealed by analyzing the differences of kinetic energy and potential energy of vibrated granular balls in the three different gravity environments. This study lays a foundation for maximizing the damping effect of vibrated granular materials with constant mass by controlling their dissipation behavior, which provides a new idea for the universal design of granular damping structures in engineering practice.

To enable future deep space exploration, orbital refueling of spacecraft is essential. However, transferring liquid in a microgravity environment is a complex process dependent on various factors. One of the basic and critical tasks is to separate phases to allow the supply of gas-free liquid from one tank to another. For this purpose, a liquid acquisition device is essential. In this work, a screen channel liquid acquisition device was designed and used to investigate phase separation and liquid removal from an experiment tank in a microgravity environment. The experiments were performed using the drop tower facility at the University of Bremen, with HFE-7500 as the test liquid under isothermal conditions. This investigation explored the interdependent effects of various phenomena, including the reorientation of liquid in the tank, capillary rise between parallel plates, flow through screen pressure variation, and bubble point breakthrough. Under subcritical conditions, the SC-LAD was found to supply gas-free liquid at the outlet, as long as the pressure drop across the screen was lower than the bubble point threshold. At the critical point, the screen started to ingest bubbles, resulting in a sharp peak in the differential pressure signal. The wetted area of the screen was obtained by analyzing images captured with a high-speed camera and used to calculate the analytical pressure drop. The experimental results were compared with the analytical solution and discussed in detail.

In the present paper, merging of two successive compound droplets in an abrupt expansion microchannel using direct numerical simulations is presented. The compound droplets undergo deformation and velocity decreases when entering the expansion region. Their interaction behaviors are divided into two modes of merging and non-merging. These two modes are dominated, and influenced by fluid dynamic parameters, compound droplets’ center distance, the expansion ratio of microchannels and the size of compound droplets, which are analyzed through the results of numerical simulation. The capillary number, the fluid viscosity, and the droplets’ distance increase lead to the merging time of the droplets increases. Although increasing the inner interfacial tension does not significantly affect the merging time of two outer droplets, it significantly reduces the merging time of two inner droplets. Meanwhile, varying the expansion ratio and the droplet size results in the transition between the two interaction modes. Two diagrams for the mode transition, based on the capillary number, the droplet center separation, and the droplet size are also given.

Abstract

This paper presents a reconfigurable satellite ground microgravity simulation system based on a parallel mechanism, which allows cxsfor adjustable gravity coefficients and can simulate three-dimensional space movement with fast response and high accuracy. Firstly, the parallel motion platform and parallel six-dimensional force sensor designed specifically for the microgravity simulation system serve as the mechanical structure of the system. Secondly, a control system for simulating microgravity has been proposed, which includes a data acquisition component and a motion control component. Thirdly, a novel microgravity simulation algorithm, which can adjust the gravity coefficient and is based on the constant variation method, was proposed to establish the mapping relationship between the six-dimensional external force and displacement. Finally, the six-dimensional force sensor is statically calibrated and demonstrated excellent measurement performance. After implementing gravity compensation through surface polynomial fitting, the motion platform for microgravity simulation can react within 0.15 s upon detection of a force signal by the sensor, with a response error of less than 3%. The ground microgravity simulation system based on parallel mechanisms has been successfully applied to test the tolerance capability of reconfigurable satellite docking interfaces.